This invention relates to secondary (rechargeable) lithium batteries which utilize thin film intercalation compounds, principally as the positive electrode. In particular, the invention provides means for fabricating such battery electrodes as thin films of lithiated ternary transition metal oxides, including LiMn.sub.2 O.sub.4, LiCoO.sub.2, and LiNiO.sub.2.
Rapid growth in the use of electronics instrumentation ranging from sophisticated telecommunication equipment and computers to audio-visual systems, watches, and toys has generated a wide-spread requirement for electronic circuits that include devices having their own power sources and energy storage. Therefore, there is a critical need for low-cost, miniaturized, rechargeable energy storage devices (batteries) that have high energy densities and can deliver power reliably at a constant voltage over many recharge cycles. As an additional requirement for most practical applications, the fabrication of these secondary batteries must be compatible with microelectronics technologies in order that such power sources may be fully integrated into complex microcircuits.
Thin-film, multilayer heterostructure systems including compounds capable of intercalating lithium ions have thus far offered the most promise of meeting the need for miniaturized secondary batteries. For example, Meunier et al., in Mat. Sci. and Eng., B3 (1989) 19-23, describe such layered structures that include TiS.sub.2 or TiS.sub.x O.sub.Y positive electrode intercalation compounds with elemental lithium negative electrodes. These materials provide only about 1.25 to 2.6 V at a 1 microamp/cm.sup.2 current density, however. A similar lithium anode thin film cell described in U.S. Pat. No. 4,751,159 employs AgMo.sub.6 S.sub.8 as the positive electrode intercalation material and is reportedly capable of providing voltages of about 1.4 to 3 V at a current density of 300 microamps/cm.sup.2. Although this intercalation cathode compound shows improving performance capability, thin film battery composites continue to suffer from the disadvantage of depending upon dangerously reactive lithium metal anodes as the Li.sup.+ ion source.
Further improved performance with open circuit voltages in the range of 4 V at energy densities of 200 to 500 microamps/cm.sup.2 has been exhibited by secondary battery cells having bulk, pelletized positive intercalation electrodes of three-dimensional, spinel-structured LiMn.sub.2 O.sub.4 (U.S. Pat. No. 4,828,834), and layered LiCoO.sub.2 (Mizushima et al., Mat. Res. Bull., 15, 783 (1980)) and LiNiO.sub.2 (Dahn et al., Solid State Ionics, 44, 87 (1990)). These materials exhibit the additional benefits of being light weight and providing a source of lithium ions that enables substitution of similar or other intercalatable materials, e.g., graphite, WO.sub.2, and Al, for the environmentally undesirable lithium metal anode.
Unfortunately, however, these lithiated transition metal oxides have properties that until now have detracted from their serious consideration as candidates in thin film fabrication processes with widely-used electronic component materials such as GaAs and silicon. Initially, the great disparity between the melting points and atomic masses of lithium and the transition metal constituent would ordinarily prevent stoichiometric deposition from a bulk intercalation compound source in commonly-employed fabrication processes such as reactive electron beam evaporation. Further, the high temperatures, often in excess of 800.degree. C., at which these intercalation compounds are normally crystallized in bulk are inimical to their incorporation into microcircuits with GaAs decomposing above 350.degree. C. or Si which deteriorates above about 500.degree. C. Such high temperature processing of these lithiated ternary metal oxides has also been found to produce crystallite grain sizes generally larger than about one micrometer, thereby severely limiting the electrode surface area, and thus the intercalation kinetics, in typical 0.5 to 1.5 micrometer thin films.
In the present invention, we have found the means to avoid these disadvantages and to fabricate lithiated ternary transition metal oxide thin film intercalation electrodes for secondary batteries under conditions that are compatible with microelectronics technology and that produce high electrode surface area and resulting exceptional performance.